It is known, for example from WO 98/26948, EP 0 512 358 and U.S. Pat. No. 5,529,324, to provide roll control systems. In particular it is known from U.S. Pat. No. 5,529,324 to disclose a vehicle suspension system comprising a hydraulic actuator, a source of hydraulic pressure and a drain, a pressure control valve for controlling the flow of hydraulic fluid from the source to the drain thereby to control the hydraulic pressure drop between the source and the drain, at least one actuation control valve for connecting the actuator between the source and the drain such that said pressure drop controls the magnitude of an actuating force produced by the actuator, and control means arranged to operate the valves in response to at least one input defining a desired actuating force.
However it can be a problem with such systems that the pressure control valve, even when fully open, creates a minimum pressure drop across itself defined partly by the cross section of the flow path through the valve when open and partly the viscosity of the fluid. Therefore a minimum pressure which can be applied across the actuator and a minimum roll control correction force which can therefore be applied. The present invention aims to minimize this problem.